Many electrical systems have circuits that use inductor and capacitor tanks to provide a fixed frequency output from a voltage source. These outputs are used in a variety of applications, including radio transmissions and radio frequency generations.
In many of these systems, the loading characteristics of a tank are substantially fixed. When the load is fixed and constant, a fixed drive level provides a constant amplitude of energy to the tank. In addition, if a system has a constant voltage power source, low impedance bridge circuits provide a stable and repeatable drive level.
However, when the load is variable, a constant drive level does not provide a constant power amplitude level to the tank. Moreover, if the voltage source of a system is not constant, bridge circuits do not provide a constant drive level.
In addition, in systems that are battery operated, a power inverter is typically used to provide the constant voltage source because the voltage level drops as the life of the battery increases. Unfortunately, power losses typically are associated with the use of such power inverters as the power inverter attempts to stabilize the variable battery voltage.
For example, in the prior art half-bridge circuit of FIG. 1, the power source is applied directly to the series inductor-capacitor (LC) tank. The half-bridge configuration has an input and an output divided between two identical devices, each operating in phase opposition with respect to the other. In this instance, the output is the tank which is center tapped with respect to a P-channel metal oxide semiconductor field effect transistor (MOSFET) and an N-channel MOSFET. Thus, one side of the circuit carries current from battery to ground through the tank, and the other side of the circuit operates in phase opposition. However, as the battery life increases, the power delivered to the tank will vary and decrease, thus changing and decreasing the power radiated from the tank.
In the prior art system of FIG. 2, a full-bridge circuit uses an inverter to alternately provide power to the tank. However, the power level merely is alternated, and the power drive level to the tank is not controlled or regulated to provide a constant drive level. Therefore, the same power loss problem occurs in the system of FIG. 2 when the battery life increases.
Thus, a system and method are required to provide a constant drive level for a tank in a drive circuit. An improved system is needed to modulate the power provided to the tank of the drive circuit and to control the modulation to provide a fixed drive level when variations in the power level of the power source exist or when variations in the tank load exist.